Japanese Patent Laid-open No. 94334/1981 to the present applicant discloses a basic automatic focusing device of the same general type as the present invention. The focusing device of this patent teaches the use of an electric signal which changes with respect to the varying position of a photographic lens passing light from an object. If the electric signal contains information about the direction in which the photographic lens is to be changed, then the photographic lens is moved in that direction to an optimum focus. If not signal containing information about the direction of movement is obtained with a change in the position of the photographic lens, a lens driving motor is rotated at a high speed. In this case, the photographic lens is thus moved between an infinite distance photographing position and a least distance photographing position until a signal containing information about the direction of movement is obtained.
One disadvantage of this known focusing arrangement, however, arises in the case when a signal containing information about the direction of movement is not obtained with a change in position of the photographic lens, i.e., in the case when the contrast of the object image is very low or in the case of an object image having no contrast such as a white object.
In such a case, the photographic lens is in an insensitive region, in which the detection of contrast is impossible. Therefore, no electric signal which changes with a changing position as mentioned above, can be obtained even if the photographic lens is reciprocated between the infinite and least distance photographic positions any number of times. During this time, the lens driving motor is rotated at a high speed, thus causing excessive loads on the motor and the drive mechanism therefor. Further, when this focusing device is used in conjunction with a television camera, even a low contrast object image light beam passes through the photographic lens so that the image itself is reproduced on the picture tube. The viewer thus inevitably watches unpleasant out-of-focus images according to the movement of the photographic lens.